RECOVERY MECHANISM OF THE REACTION INTERMEDIATE PRODUCED BY PHOTOINDUCED CLEAVAGE OF THE INTRAMOLECULAR HYDROGEN-BOND OF DIBENZOYLMETHANE

The recovery mechanism of the reaction intermediate (non-chelated enol form) produced by photoinduced cleavage of the intramolecular hydroge n bond of dibenzoylmethane was studied in various solvents by nanoseco nd laser flash photolysis. The recovery rate and mechanism depend stro ngly on the nature of the solvent. Unimolecular recovery of the interm ediate to the chelated enol form takes place in acetonitrile, diethyl ether and dimethylsulphoxide with extremely small rate constants (1.1, 1.5 and 6.6 s(-1) respectively) despite the small activation energy ( 3.6 kcal mol(-1) in 3-methylpentane). The slow unimolecular recovery r ate can be ascribed to the small frequency factor (7.0 X 10(5) s(-1)), i.e. the large negative entropy change for the formation of the chela ted enol form. In non-polar aliphatic hydrocarbon solvents, a bimolecu lar recovery process via hydrogen-bond interactions between two interm ediate molecules is included in addition to unimolecular recovery. In alcohols, a solvent-assisted recovery process by mutual hydrogen excha nge between the intermediate and alcohol molecule(s) accelerates the r ecovery rate. Basic catalysts, e.g. KOH in ethanol and triethylamine i n acetonitrile, increase the recovery rate considerably by an addition al process through the enolate anion.